In recent years, development has been underway of a hydraulic circuit (defined as a closed circuit) in a work machine such as a hydraulic excavator, the hydraulic circuit being connected so as to have fewer throttle elements for driving a hydraulic actuator and supply hydraulic fluid from a hydraulic drive source such as a hydraulic pump to the hydraulic actuator before returning the worked hydraulic fluid to the hydraulic drive source without feeding the fluid back to a tank so that the rate of fuel consumption may be lowered.
On many work machines, a single rod type cylinder is used as a hydraulic actuator. In the single rod type cylinder, the pressure-receiving area of the internal piston on the head side is different from that on the rod side. It follows that with the cylinder connected to a closed circuit, driving the piston causes excess or shortage of the flow rate of hydraulic fluid within the circuit. There exists a closed hydraulic circuit furnished with a flushing valve to control such excess or shortage of the flow rate of hydraulic fluid (e.g., see Patent Literature 1).
There is also provided a driving device for a work machine, the driving device being capable of supplying optimum power in accordance with a load and including: a closed circuit that controls the operating speed of a hydraulic pressure actuator connected to a hydraulic pressure pump through variable displacement control of the hydraulic pressure pump of which the flow rate is controlled by variable displacement device; an open circuit that controls the operating speed of the hydraulic pressure actuator connected to a control valve through variable displacement control of the hydraulic pressure pump of which the flow rate is controlled by variable displacement device different from the above variable displacement device that controls the flow rate of the hydraulic pressure pump in the closed circuit and through flow rate control effected by the control valve for controlling hydraulic fluid supplied from the hydraulic pressure pump and by a bypass valve furnished in parallel with the control valve; and a distribution circuit that distributes the hydraulic fluid from the hydraulic pressure pump in the open circuit to the hydraulic pressure actuator in the closed circuit (e.g., see Patent Literature 2).